1. Field of the Invention
The present invention relates to an electron beam writing method and a fine pattern writing system for writing a fine pattern according to a desired uneven pattern when manufacturing an imprint mold, magnetic transfer master substrate, or the like for a high density magnetic recording medium, such as a discrete track medium, bit pattern medium, or the like.
The invention also relates to a method for manufacturing an uneven pattern carrying substrate, including an imprint mold, magnetic transfer master substrate or the like, having an uneven pattern surface formed through a writing step performed by the electron beam writing method described above. The invention further relates to a method for manufacturing a magnetic disk medium having an uneven pattern transferred thereto from the uneven pattern carrying substrate or imprint mold, and a method for manufacturing a magnetic disk medium having a magnetic pattern transferred thereto from the magnetic transfer master substrate.
2. Description of the Related Art
Generally, information patterns, such as servo patterns and the like are formed on current magnetic disk media. In view of the demand of higher recording density, a discrete track medium (DTM) in which magnetic interference between adjacent data tracks is reduced by separating the tracks with a groove pattern (guard band) has been attracting wide attention. A bit pattern medium (BPM) proposed for achieving still higher density is a medium in which magnetic substances forming single magnetic domains (single-domain particles) are physically isolated and disposed regularly, and one bit is recorded in a single particle.
Heretofore, fine patterns, such as servo patterns and the like, have been formed on magnetic media by uneven patterns, magnetic patterns, or the like and an electron beam writing method for patterning a predetermined fine pattern on a master of a magnetic transfer master substrate or the like has been proposed. In the electron beam writing method, a pattern is written on a substrate applied with a resist by irradiating thereon an electron beam corresponding to the shape of the pattern while rotating the substrate as described, for example, in U.S. Pat. No. 7,026,098 and Japanese Unexamined Patent Publication No. 2006-134924.
The electron beam writing method described in U.S. Pat. No. 7,026,098 is a method in which when, for example, writing a rectangular or parallelogram element constituting a servo pattern extending in the width direction of a track, the electron beam is deflected in a radial direction while being vibrated rapidly in a circumferential direction, thereby scanning the beam so as to completely fill the area of the element.
Japanese Unexamined Patent Publication No. 2006-184924 discloses, as on/off writing method, a method in which pattern writing is performed by on/off irradiating an electron beam on a substrate applied with a resist according to the shape of a pattern while rotating the substrate, and shifting the substrate or electron beam irradiation unit by one beam width every rotation of the substrate in a radial direction. It also discloses an electron beam writing method in which the electron beam is vibrated back and forth in a track width direction of a pattern. It further discloses a method for controlling write start timing of a servo pattern.
In the control method disclosed in Japanese Unexamined Patent Publication No. 2006-184924, the write start timing of a servo pattern is controlled such that the writing is started after a predetermined standby time from reference signal S1. The reference signal is preferable to be outputted as many as a number of servo areas to be written in one rotation, and the write start timing of the servo pattern in each servo area is determined based on the reference signal outputted according to each servo area. More specifically, the timing control is thought to be like that shown in FIGS. 12 and 13. That is, for example, where a hard disk pattern is a pattern having 8 servo areas 12A, 12B - - - 12H, as shown in FIG. 12, the writing unit is configured to output 8 reference signals S1, S2, - - - S8 during one rotation, and the writing is started in each of servo areas 12A, 12B - - - 12H based on each reference signal outputted according to each servo area. More specifically, the writing of servo area 12A is started based on reference signal S1, and the writing of servo area 12B is started based on reference signal S2. Generally, the servo area is an area extending from the inner circumferential side to the outer circumferential side in an arc, and the write start position of the servo pattern in a circumferential direction is different depending on the position in the radial direction (track), as illustrated in FIG. 12. That is, as illustrated in FIG. 12, the write start positions for the pattern in servo area 12A for radial direction positions ra, rb, rc are pa, pb, and pc respectively, and the positions thereof in the circumferential direction (rotational direction positions) are different from each other. Here, according to the description of Japanese Unexamined Patent Publication No. 2006-184924, the writing is started for radial direction positions ra, rb, rc after predetermined standby times (ta, tb and tc respectively) from reference signal S1, as illustrated in FIG. 13. Accordingly, depending on the distance from the position of reference signal S1 to the write start position, the standby time from reference signal S1 becomes long or short. That is, there is a large difference between the standby time to start writing at a radial direction position with a minimum distance from the position of reference signal S1 to the write start position and the standby time to start writing at a radial direction position with a maximum distance from the position of reference signal S1 to the write start position. Unlike the number of servo areas and the curvature of the arc typically employed, FIG. 12 illustrates less number of servo areas (8 areas) with a larger curvature for the arc for clarity and facilitating understanding.
The inventors of the present invention have studied the relationship between the number of pulses from predetermined encoder pulses (only from A-phase encoder pulses) and the mount of positional displacement (nm) using a rotary encoder having a predetermined number of encoder slits, the results of which is shown in FIG. 7. In FIG. 7, the horizontal axis represents the rotation amount obtained from the number of pulses (mrad), and the amount of positional displacement at each rotation amount is calculated from the encoder pulse jitter. The amounts of positional displacement are obtained for PLL control method when the rotational speed is set to 20 rpm and 30 rpm. The results have revealed that, in both of the rotational speeds, positional displacement occurs in the amounts of not less than 10 nm and not less than 20 nm when the rotation amount exceeds 30 mrad and 100 mrad from the reference measurement position respectively.
The current arc shaped servo area has a difference in rotational position up to about 70 mrad for pattern start positions. Thus, in a configuration in which writing of an arc shaped servo area is started after defined standby times from the same reference signal regardless of the radial direction position, as in Japanese Unexamined Patent Publication No. 2006-184924, the rotation amount of the farthest write start position from the reference signal is about 70 mrad, and FIG. 7 shows that the amount of positional displacement at the position is 15 to 20 nm. The positional displacement of about 15 to 20 nm will not pose any significant problem if a bit length exceeds 200 nm, but when the bit length becomes on the order of not greater than 100 nm in response to the demand of higher recording density, the positional displacement of 15 to 20 nm will lead to the degradation of tracking accuracy.
The present invention has been developed in view of the circumstances described above, and it is an object of the present invention to provide an electron beam writing method and fine pattern writing system capable of improving the accuracy of write start position of a pattern.
It is a further object of the present invention to provide a method for manufacturing an uneven pattern carrying substrate, such as imprint mold, magnetic transfer master substrate, or the like, having a pattern accurately written by an electron beam. It is a still further object of the present invention to provide a method for manufacturing a magnetic disk medium having an uneven pattern or a magnetic pattern transferred thereto using the uneven pattern carrying substrate.